iMessage to android with random person respond

My friends phone number is on an android and every time I text them it goes in as a iMessage and they can’t see it he inquired with T-Mobile and they said the last person with the phone number had an iPhone OK well today we were messaging and I was sending him through iMessage and requesting to send in text. When a random person started messaging back with iMessage spamming in on the conversation? What’s going on why would this be happening is this possible that either the email is hacked or the phone number through Apple? How do we stop it Without having to change the phone number or just for added security so this doesn’t happen again my friend really wants to get an iPhone but is afraid this person will be on their iMessage as well.

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